Salt (NaCl) is the chemical which is most commonly employed to deice roads, driveways, sidewalks and the like. Salt is as low as, or lower in cost, than any other chemical which is presently available for use as a deicer. The reasons for this are its high weight efficiency as a deicer and its wide availability in a purity which is sufficient for use after minimal processing.
The costs associated with purchasing and applying ground "rock salt" are misleading with respect to the total cost of this material as a road deicer. The aqueous solutions of sodium chloride which result from melting ice and snow severely damage vehicles and highway structures by corrosion of their metal parts, and harm flora, fauna, soil and water supplies. The Environmental Protection Agency has released data suggesting that the cost of this damage is fourteen times the direct cost of deicing with sodium chloride. D. M. Murray et al., An Economic Analysis of the Environmental Impact of Highway Deicing, U.S. Environmental Protection Agency, Cincinnati, Ohio, EPA-600/a-76-105 (May 1976).
When a ferrous metal is contacted with aerated water which is slightly acid to slightly alkaline (pH 6.5-8.3) local anode and cathode sites develop on its surface. These sites or cells, formed as a result of impurities or intermolecular stresses in the iron, have a potential of -0.41 volt, which sponsors a current flow through the base metal and the electrolyte (water). At the anode, iron is converted from the metallic to the ionic state, while at the cathode either ionic hydrogen or dissolved oxygen is reduced, causing the surface of the cathode to become more alkaline. These reactions can be represented by the following formulas:
(A) (Anode reaction)
Fe.fwdarw.Fe.sup.++ +2e. PA1 (1) Oxygen reduction PA1 (2) Ion reduction PA1 Fe.sup.++ +2HO.sup.- .fwdarw.Fe(OH).sub.2 PA1 2Fe(OH).sub.2 +1/20.sub.2 +H.sub.2 O.fwdarw.2Fe(OH).sub.3
(B) (Cathode Reactions)
1/20.sub.2 +H.sub.2 O.fwdarw.2e..fwdarw.2OH.sup.- PA2 2e.+2H.sub.2 O.fwdarw.2OH.sup.- +H.sub.2
(C) (Combining of the two electrode products yields ferrous hydroxide), which is further oxidized to ferric hydroxide.
The deposition of Fe(OH).sub.2 and Fe(OH).sub.3 at anodic sites leads to the tuberculation and pitting of the metal surface, and the spalling of concrete from highway structures. Deicing processes, using sodium chloride as the active ingredient, contribute to these corrosion-causing reactions by increasing the conductivity of the water formed from the melted snow or ice. The dissolved chloride ion also permeates the tubercules, thus increasing their conductivity and the overall corrosion rate.
Therefore, a need exists for a method to inhibit the metal corrosion and environmental damage caused by salt-based deicing compositions. A further need exists for a composition which is an effective deicer, but which is inhibited in its ability to corrode ferrous metals when dissolved in meltwater.